D.A. Carson in The Expositor's Bible Commentary on Matthew, Vol 2 says this in a point concerning Matthew 13:13 (pp 309-310)...
This sheds much light on the parables. It is naive to say Jesus spoke them so that everyone might more easily grasp the truth, and it is simplistic to say that the sole function of parables to outsiders was to condemn them. If Jesus simply wished to hide the truth from outsiders, he need never have spoken to them. His concern for mission (9:35-38; 10:1-10; 28:16-20) excludes that idea. So he must preach without casting his pearls before pigs (7:6). He does so in parables: i.e., in such a way as to harden and reject those who are hard of heart and to enlighten--often with further explanation--his disciples. His disciples, it must be remembered, are not just the Twelve but those who were following him and who, it is hoped, go on to do the will of the Father (12:50) and do not end up blaspheming the Spirit (12:30-32) or being ensnared by evil more thoroughly than before (12:43-45). Thus the parables spoken to the crowds do not simply convey information, nor mask it, but challenge the hearers. They do not convey esoteric content only the initiated can fathom but present the claims of the inaugurated kingdom and the prospects of its apocalyptic culmination in such a way that its implications are spelled out for those in the audience with eyes to see.
